BlackBox
The black box testing approach of an alexa skill is really complicated, it literally means: try vocally, all the possible inputs that your skill can (and can't) manage.
You can proceed in this way, of course, but you should NOT, it is an insane amount of work.

For the black box testing you can use a compatible echo device or a echo simulator: https://echosim.io/

Advice #1: Separate the code
The first advice I can give is to separate the business logic from the presentation: separate the code
that exposes the Intents of your skill from the function that answer to those Intents.

If you read the code of my skill you will see that index.js contains just the intents and the imports of
the modules.
The main code of the skill is in EuroUtils.js.
This simplify debugging and it helps to write unit testing code as you can see in test.js.

Instead of a single file, like test.js for unit testing, it is better to use one of the available Unit Testing framework for nodejs.
If you have nodejs installed you can test EuroUtils.js launching node test.js inside the src folder.

Advice #2: Utterances generations
Another thing I didn't liked so much in Alexa, it is the way you have to create utterances.txt, because:
- it is too confusing
- it is hard to remember the logical link between Utterances and Intents

The more complex is your skill, the more complicated it becomes to keep track of them.
In order to improve the logical link between the two things I invented a new file, an xml, with some tags that helped to explain which Intents answers to a set of utterances.
There is an example here: utterances_groups_sample.xml

I than created a simple python script to extract and print on console: all the utterances to submit for the skill.